The present invention relates in general to reproducing images on a medium having a conductive base substrate and a dielectric layer, and more particular to an electrographic printing device for use in data processing.
Various processes and techniques of image reproduction are found in the prior art, which are concerned with the electrostatic transfer of a charge onto a print medium. In such electrographic printing systems, including the system herein disclosed, a latent image is formed on a dielectric medium by placing the medium in the field established between two electrodes. The two opposed electrodes have a high electrical potential difference supplied across them, thereby establishing the necessary field across the direction medium. The latent charge image formed on the medium is in the shape of the electrode surface that faces the dielectric surface of the medium.
There are various types of electrographic printing systems, one of which employs character shaped electrodes and wherein a print drum is rotated at high speed and selected electrodes are pulsed when the desired character is facing the dielectric surface, causing the formation of latent images on the medium at the area where the character electrode is located.
Another system employed in prior art devices is the pin type of system wherein various patterns such as alpha-numeric characters are reproduced by selecting predetermined electrodes, as the recording medium passes the electrodes.
A third type of system is disclosed in U.S. Pat. No. 3,624,661 entitled "Electrographic Printing Systems With Plural Staggered Electrode Rows" issued Nov. 30, 1971 to Shebanow and Borelli, and assigned to the assignee of the present invention. This system provides a multiple row electrode structure wherein successive rows are mutually spaced from each other, each row including mutually spaced electrodes with the electrodes of successive rows being positioned in a staggered manner, with respect to each other. A drive circuit is also disclosed in the aforementioned patent wherein a plurality of output lines coupled the matrix to the electrodes so as to selectively apply a high voltage to the electrodes to produce a latent image on a dielectric medium. The apparatus set forth in that patent, through the use of a dual row set of styli which when energized provides a means of printing high quality matrix characters, is found to be a distinct improvement over prior art electrographic printing apparatus and has achieved good results.
However, in the disclosure of the above-referred to patent, the print head consists of offset styli of equal cross-sectional area, as well as providing for equal spaces between adjacent styli and an equivalent spacing between rows. Printing is accomplished by electrically energizing the required styli necessary to form the shape of the desired character as the electrographic medium is transported over the print head. The conventional electrographic process is used to form the latent image (charged image) on the dielectric medium surface, which is later developed in a toning station.
Therefore, to form a completely solid character, the first row must be electrically energized first, and the second row energized only when the dots printed in the first row move in between the styli of row two. While the use of the substantially square styli at relatively large spacing has proved satisfactory, it has been found that by reducing the cross-sectional area of the styli, through a change in the mode of operation of the printing technique, faster printing will result and lower voltages can be used to generate a high quality of character printing.
The present invention therefore has as an object to provide a print head of the type described which is simple to manufacture and having styli of reduced capacitance thereby reducing the circuit loads for the styli circuit drivers.
Another object of the present invention is to reduce the possiblity of styli destruction from capacitance discharge by providing apparatus where less energy is stored between the styli.
A further object of the present invention is to reduce the styli cross-sectional area thereby lowering the voltage required on each stylus to establish an electrostatic field for printing and increasing the printing speed, while maintaining high print quality.